There is evidence that bone mass is increased in black as compared to white individuals so that there is less risk of osteoporosis and associated fractures. The increased skeletal mass is attributed to increased muscle mass. Further, urinary calcium is lower in blacks than in whites. It is proposed that the increase in bone mass in blacks results from skeletal resistance to parathyroid hormone (PTH) and possibly to 1,25-dihydroxyvitamin D [1-25(OH)2D] and that diminished urinary calcium caused by consequent increases in circulating PTH. Preliminary results support this hypothesis: young adult blacks (age 20-35 years) show significant increases in mean serum immunoreactive PTH, serum 1,25(OH)2D and urinary cyclic AMP and significant decreases in mean urinary calcium as compared to age-matched whites. Mean serum calcium, serum ionized calcium, serum phosphorus and creatinine clearance are the same in the two groups. In view of these observations, additional studies are proposed a) to establish that the skeleton of blacks is resistant to PTH by determining the response to human synthetic PTH (1-34) and whether intestinal absorption of calcium is increased in blacks because of increased circulating 1,25(OH)2D, b) to determine whether the skeleton of blacks is resistant to 1,25(OH)2D3 and whether binding and uptake of 3H-1,25(OH)2D3 by cultured skin fibroblasts are diminished in blacks, c) to determine the age at which increases in bone mass and skeletal resistance to PTH become evident in blacks, d) to determine the effects of aging on bone mass at 3 sites in blacks, forearm, lumbar spine and hip, and whether there is an age-related loss of renal production of 1,25(OH)2D and impairment in ability to adapt to a low calcium intake in blacks as occur in whites, e) to determine the influence of the menopause on vitamin D and mineral metabolism in black women, and f) to determine the effects of increased body weight in obese subjects and of increased muscle mass in body builders and weight lifters on bone mass and vitamin D and mineral metabolism. The proposed work should provide new information about why the skeleton is resistant to the development of osteoporosis and urinary calcium is reduced in blacks and whether increased strain produced by gravitational forces can influence bone mass and produce alterations in the vitamin D-endocrine system similar to those observed in blacks. The results could also lead to new approaches for investigating the pathogenesis and treatment of osteoporosis.